In a mobile communication system of the LTE (Long Term Evolution) type specified by the 3GPP, tracking area lists (hereinafter referred to as the TA list) have been considered to be managed by use of multiple layers in order to avoid a significant increase in location registration processing load at an exchange MME. Such an increase in processing load is caused by concurrent transmission of location registration (TAU: Tracking Area Update) signals in a case where a number of users of mobile stations are on the same train.
To be more specific, as shown in FIG. 1, such a mobile communication system is configured as follows. The mobile stations are divided into multiple layers so that tracking areas (hereinafter referred to as TAs) included in TA lists assigned to a mobile station #a belonging to a first layer are different from TAs included in TA lists assigned to a mobile station #b belonging to a second layer.
Here, each of the mobile stations #a and #b is configured to transmit a TAU signal, when detecting that the mobile station has moved out of TAs included in the TA list with which the mobile station is registered by location registration processing.
Therefore, even in such a case where users of the mobile stations #a and #b are on the same train as described above, timing for transmitting the TAU signal varies between the mobile stations belonging to the respective layers. Thus, a significant increase in location registration processing load at an exchange MME can be avoided.
However, in the existing LTE type mobile communication system, the TA list cannot be set to cover different pool areas controlled by the exchanges MME, respectively. Therefore, when mobile stations move between the pool areas, the TAU signals are transmitted all together at the same timing. This has resulted in a problem of significantly increasing the location registration processing load at the exchange MME.
To be more specific, as shown in FIG. 1, when the mobile stations #a and #b move beyond a boundary 1 between a pool area #1 and a pool area #2 or beyond a boundary 2 between the pool area #2 and a pool area #3, both of the mobile stations transmit the TAU signals at a timing T1 or T2. This causes a problem of leading to a situation where the location registration processing load at the exchange MME is significantly increased.